Setting

AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA

REGIONAL PALEOGEOGRAPHY

TECTONO-STRATIGRAPHIC SETTING

From the Cambrian through the Early Mississippian, central Nevada lay within a north-south trending geocline, a broad and gentle depositional trough, representing the shelf and slope bordering the passive or tectonically inactive continental margin of western North America. This Cordilleran geocline or miogeocline was probably initiated during Late Precambrian rifting of the north-trending continental margin of western North America which lay in central Nevada. Clark and others (1985) suggest that the Precambrian McCoy Creek Group rocks exposed in the Bull Run Mountains of northern Elko County provide stratigraphic evidence of this Cordilleran rifting event.

Rocks of the Lower Paleozoic Eastern Carbonate Assemblage on the continental platform graded successively westward and downslope through an abrupt transitional carbonate-clastic assemblage representing outer shelf and upper slope depositional environments, and into a coeval siliceous and volcanic assemblage which formed in an oceanic island-arc setting. The Lower Paleozoic Eastern Carbonate Assemblage is composed of limestone and dolomite, quartzite, and shale; the Lower Paleozoic Siliceous and Volcanic Western Assemblage is a heterogenous lithologic package with chert and quartzite the most common units and lesser amounts of siltstone and sandstone, conglomerate, shale, limestone, and mafic flow and pyroclastic rocks; the Lower Paleozoic Transitional Assemblage is lithologically intermediate and contains stratigraphic elements common to, as well as locally interfingering with, both the eastern carbonate and western siliceous and volcanic assemblages. The transitional assemblage in many cases is very similar to western assemblage rocks with a higher percentage of carbonate and finer siliceous detritus (Ketner, 1977b).

The sinuous north-trending hingeline representing the Paleozoic margin of cratonal North America was developed as a shelf-slope break which lay at the approximate longitude of the western Carlin-Pinon and Sulphur Spring Ranges from the Late Ordovican through the Early Devonian. Beginning in the Late Devonian, basinal shales, cherts and limestones of the western and transitional assemblages were structurally juxtaposed and emplaced over the thick carbonate shelf which formed the irregular margin of North America. This emplacement, the result of the Antler orogeny, was probably the effect of the collision of a migrating island-arc with the passive margin of North America (Moores, 1970; Johnson and Pendergast, 1981; Speed and Sleep, 1982).

Devonian through Mississippian "Antler" thrusting compressed original facies about 45 to 75 miles to create the Roberts Mountains allochthon. The upper plate of the Roberts Mountains thrust, composed of many small thrust plates, forms a sheet of western assemblage rocks which overlie, and are occasionally eroded through, to expose underlying autochthonous eastern carbonate assemblage rocks. Structural/stratigraphic relationships suggest this allochthon represents a pre-assembled accretionary sheet deposited along the continental slope or rise of another crustal plate which collided with, and was obducted onto, the western margin of North America (Johnson and Pendergast, 1981; Speed and Sleep, 1982). Emplacement was probably accomplished within about 5 m.y. based upon convergence rates of about 6.5 centimeters per year (Johnson and Pendergast, 1981; Dickinson and others, 1982).

This eroded and structurally segmented allochthon is now exposed at Battle Mountain, in the Monitor, Fish Creek, Antelope, Toquima, Toiyabe, Shoshone, Sulphur Spring, Carlin-Pinon and Adobe Ranges, the Dry Hills, the Cortez, Roberts, Independence and Tuscarora Mountains as well as to the northeast in the Bull Run Mountains, the Owyhee, Mountain City and Rowland Quadrangles, Marys River Valley area, the HD Range, Windermere Hills, and Snake Mountains. Bending and offset of the trace of the Roberts Mountains thrust in northeastern Nevada is probably in large measure a reflection of the original irregular shelf margin modified to some degree by later Mesozoic thrusting and Cenozoic low-angle normal faulting and strike-slip faulting.

An elongate asymetric foreland basin, the Antler foreland, was created and partially overridden during initial tectonic loading and downflexing of the continental shelf by the Roberts Mountains allochthon (Johnson and Pendergast, 1981; Speed and Sleep, 1982). This 120 mile wide basin was filled with a thick Mississippian through Permian sequence derived from the west which we have termed the Upper Paleozoic Transitional Detrital-Carbonate Assemblage. This assemblage primarily represents shallow to deep-marine fan and fan delta sediments eroded from the steep sinuous face of the allochthon.

Initial filling of the foreland may have began in Late Devonian with depostion of the Pilot Formation and was certainly underway by the Early Mississippian with deposition of the Chainman Formation. During the Pennsylvanian and throughout much of the Permian, this foreland was blanketed with shoal-water carbonates and minor clastics in several local depositional basins.

Oversedimentation during the Mississippian and Pennsylvanian created shallow evaporite basins begining in the Permian. Although restricted primarily to the eastern portion of the study area, the Permian Butte-Deep Creek depocenter received as much as 15,000 feet of Permian sediments. In the western portion of the study area, contemperaneous Permian conglomerates and minor sands and shales were deposited in local depositional basins which received a few thousand feet of sediments.

The Mesozoic was a time of retreating or regressing seas across northeastern Nevada with Lower-Triassic marine shales and limestones, succeeded by Upper-Triassic and lower Jurassic fresh water and eolian sediments. Small patches of Cretaceous sediments exist across the study area and primarily represent continental deposition. As much as 30,000 to 40,000 feet of strata accumulated in northeastern Nevada from the Precambrian through the Triassic.

The Late Permian through Early Triassic Sonoma orogeny emplaced siliceous and volcanic sediments of Havallah and Pumpernickle Formations to the west of, and partially over, thin upper Paleozoic shallow-water carbonate-detrital rocks which overlap the Roberts Mountains allochthon (Silberling and Roberts, 1962; Speed, 1971; Kleinhampl and Ziony, 1985). The Golconda allochthon is primarily exposed to the west of the evaluation area, but is present in the western margin of the area in the Shoshone and Toiyabe and Monitor Ranges, as far north and east as the Independence Mountains, in the eastern Owyhee and western Mountain City Quadrangles, and the Elk Mountains portion of the Jarbidge Wilderness. The Golconda allochthon obscures stratigraphic relationships along the western margin of the Roberts Mountains allochthon. Its emplacement has folded and telescoped facies along the western margin of the evaluation area.

Eastward directed, dominantly Late Cretaceous thrusting related to the Sevier Orogeny, is well documented in southern Nevada and western Utah where Upper Precambrian and Lower Paleozoic rocks are emplaced over Upper Paleozoic and lower Mesozoic rocks (Burchfiel and Davis, 1972). This thrusting has been invoked by many workers to explain structural relationships in several ranges in northeastern Nevada (Hazzard and others, 1953; Misch and Easton, 1954; Nelson, 1959). Closer examination by many other workers suggest that there are very few faults which are true Mesozoic thrusts repeating section and placing older on younger rocks (Armstrong, 1966, 1972; Moores and others, 1968; Hose and Blake, 1976; Miller and others, 1983, and many others). Instead these faults appear to be low-angle normal faults with extensional rather than compressional origins, and place younger over older rocks with stratigraphic attenuations of several thousand feet. Exceptions are present in the northern Pancake, Quinn Canyon, Grant and Egan Ranges and in northern Elko County in the Snake Mountains, Windermere Hills, HD Range, and Pilot Range (Riva, 1962; 1970; O'Neill, 1968; Coats and Riva, 1983; Bartley and others, 1985; Fryxell, 1984). Nearly all of these faults have moderate to small stratigraphic seperation or displacements, less than 2,000 feet, but have quite apparently afffected stratigraphic recontruction.

The Cenozoic was dominated by volcanism from about 43 Ma to about 6 Ma both as ignimbritic caldera related erutions and less voluminous intermediate to mafic flows and pyroclastics. (Coats, 1985). Eocene-Oligocene eruption of voluminuous ignimbritic volcanics or ash-flow tuff sheets are ubiquitous across the study area. Caldera complexes localized eruptive centers for enormous volumes of ash-flow tuffs and then collapsed over these vents along subcircular and elliptical faults to form subsided caldrons (Kleinhampl and Ziony, 1985; Gilluly and Gates, 1965).

Along with calderas, shield domes and flow fields were developed and later partially to completely destroyed by erosion during the Cenozoic. Cenozoic Basin-Range tectonism extended the entire area at least thirty percent creating a series of grabens filled with Tertiary volcanics and clastic debris, and thick fluvio-lacustrine sediments which characterize the present physiography.

Tertiary low-angle normal fault complexes have locally created as much as 200 percent extension of the stratigraphic section and placed younger Paleozoic and Tertiary stratigraphic units over older Paleozoic and Tertiary strata in the Pancake, Quinn Canyon, Egan, Grant, Schell Creek, Snake, Bristol, Wood Hills, Ruby, Pilot, and Goshute Ranges. The timing of this deformation is imprecise and much of the low-angle younger over older normal fault deformation has been, and still is, incorrectly attributed to Late Jurassic through Cretaceous Sevier thrusting which has locally placed older Mesozoic and Paleozoic rocks over younger Mesozoic and Paleozoic strata.